This invention relates to transparent coatings and methods to prepare the coatings. More specifically, this application relates to optically clear abrasion (or scratch) and chemical-resistant coatings for use on plastic surfaces. These coatings can also be used on metallic substrates for improving their hardness and anti-corrosion (or barrier) properties.
The present invention fulfills the need for abrasion or scratch-resistant coatings on plastic substrates, which are needed in a variety of applications such as, ophthalmic and sportswear lenses, automobile and airplane windows. Plastic substrates, such as polycarbonate and acrylic, can scratch easily and lose transparency quickly during daily use and maintenance. Hard and optically transparent coatings for plastic substrates possess significant market potential. To date, however, a totally successful abrasion-resistant coating technology is not yet available.
Optical transparent coatings on polymeric substrates have been developed using two approaches: wet chemical methods (e.g., sol-gel processes), and vacuum/gas phase processes. The latter approach is based on the deposition of an inorganic material by a plasma torch. Using this approach, high abrasion resistance can be obtained, but in addition to high cost, there is often the problem of poor adhesion because of the difference in the thermal expansion coefficient of the substrate and the coating. The sol-gel process is an attractive alternative compared to vacuum/gas phase techniques due to its low cost and good coating adhesion. However, the abrasion resistance of the sol-gel derived coating in general is lower than that produced by the vacuum/gas processes. There is still a need to formulate sol-gel based abrasion and scratch-resistant coating compositions exhibiting good abrasion and scratch-resistance properties.
Sol-gel derived hard coatings on polymeric substrates typically involve the synthesis of an organic-inorganic hybrid material. The first step of the sol-gel synthesis process involves the hydrolysis and condensation of molecular precursors such as metal alkoxide, M(OR)n, leading to the formation of a three-dimensionally crosslinked oxide network. Organic groups chemically linked to the alkoxides can be homogeneously incorporated. These alkoxides are organo-substituted silicic acid esters of general formula R′nSi(OR)4-n, where the organofunctional group R′ can act as network modifiers if R′ is non-reactive; or network formers if R′ can react with itself or additional components (R′ contains vinyl, methacryl or epoxy groups, for example).
Water is an essential component of the sol-gel synthesis process since the hydrolysis reaction initiates the process. However, metal alkoxide precursors, such as those of aluminum, titanium and zirconium, exhibit a very high reactivity towards water, making polycondensation reactions in homogeneous media almost impossible to control. To overcome this problem, a “CCC” (Chemically Controlled Condensation) method was developed in the 1980s which allows precise control of hydrolysis and condensation rate by in-stiu water generation within the system (see: H. Schmidt, B. Seiferling, G. Philipp, and K. Deichmann, in Ultrastructure Processing of Advanced Ceramics, edited by J. D. Mackenzie and D. R. Ulrich (John Wiley & Sons, New York, N.Y., 1988), p. 651). The CCC method involved a partial hydrolysis and precondensation step where the starting precursors react with a smaller amount of water than the stoichiometric amount required for complete hydrolysis of all the hydrolysable groups employed. U.S. Pat. No. 4,746,366 disclosed a process, based on the CCC method, for the production of scratch-resistant coatings. According to this process, the amount of water employed for the precondensation is introduced by means of moisture-containing adsorbents, water-containing organic solvents or hydrated salts.
U.S. Pat. No. 4,754,012 discloses organoalkoxisilane/metal alkoxide sol-gel compositions prepared by a slightly different process from that in U.S. Pat. No. 4,746,366. An organoalkoxysilane is firstly partially hydrolyzed, reacted with a mixture of metal alkoxides, and the resulting composition is further hydrolyzed and condensed to form an oxide network.
U.S. Pat. Nos. 5,357,024 and 6,218,494 disclose abrasion-resistant coating compositions containing an organoalkoxysilane and a metal alkoxide with at least one alkoxide radical substituted with a chelating ligand. The reactivity of the metal alkoxide was supposedly decreased with the chelating agent so that the hydrolysis could be conducted by addition of a stoichiometric amount of water.
A common feature of the above processes is that the water has to be meticulously controlled so that the precursor solution can only be prepared in the presence of a large amount of organic solvent. This is not desirable from an environmental point of view.
Still other prior art abrasion/scratch-resistant coatings have been described in U.S. Pat. Nos. 5,134,191, 6,228,921, 6,358,612, 6,361,868, 6,737,162, 6,939,908. A common feature in these patents is that submicron or nano-scale sized inorganic particles, such as metal oxide and other ceramic-based particles having a high degree of hardness, were used together with an organic-inorganic hybrid matrix. These coatings are categorized as a “nanocomposite” in which the inorganic particulate materials (e.g., nanoparticles) are embedded in a coating matrix as a separate phase. The inorganic particulate materials, which provide the functional performance of abrasion resistance, are an essential component of the coating compositions. However, the high surface reactivity of the small particles can lead to aggregation or increased viscosity of the precursor so that the stability in storage (both shelf life and pot life) becomes an issue. Other disadvantages associated with the use of particulate materials include high cost, difficulty in maintaining a high degree of homogeneity, complicated processes involving extra steps for dispersing the particulate materials and/or removing residual insoluble particulates in the final coating composition.
In contrast to the above prior art processes, Liu and Berg (J. Liu and J. C. Berg, J. Mater. Chem., 2007, 17, 4430, hereafter the Liu publication) reported a water-based sol-gel process (hereafter the Liu process) that can be used to prepare abrasion-resistant coatings. This process involves an aqueous precursor solution prepared by adding water into a mixture of an epoxy functional organosilane with an aluminum alkoxide. The alkoxide is then hydrolyzed and the resultant hydrate is peptized to a clear transparent solution. This solution leads to a molecular composite that contains no nanoparticles and yet possesses mechanical properties better than those of a nanocomposite. Uniquely, the Liu process involved simple mixing steps and allowed the use of a large excess of water as the solvent, which is desirable from both the cost and environmental points of view.
While the water-based process is advantageous, there is an inherent drawback for coatings based on the Liu process. According to the Liu publication, the chemistry of the subject precursor involved several ring-opening reactions of the epoxide group in the organosilane. The opened epoxide can convert to poly(ethylene oxide) (Reaction I), alkyl ether (Reaction II), and diol (Reaction III) (See FIG. 1). Reaction I is a self-polymerization reaction that occurs upon drying, which leads to a high degree of cross-linking of the derived coating and is responsible for the good abrasion resistance. Reactions II and III, on the other hand, are not wanted, as their products, alkyl ether and diol, do not polymerize by themselves and thus do not participate in network formation. Nevertheless, Reactions II and III are favored in aqueous solution at pH<7, and their reaction rates increase as pH decreases. The natural pH of the final sol-gel solution according to the Liu process was 4-5, although no acid was added in the system. For this reason, the abrasion resistance is limited to a certain extent for the coatings prepared with the Liu process. With a standard abrasion test ASTM D1044 (500 gram load, CS-10F wheel, 500 cycles), the delta haze of a PMMA (poly(methyl methacrylate)) substrate coated with the Liu process was in the range of 6-10%. This level of abrasion resistance, however, is still not satisfactory to meet the requirement for demanding applications such as automobile and airplane windows.